Photothermic dry silver coatings stabilized with halogen-containing organic oxidizing agents

ABSTRACT

LIGHT-SENSITIVE HEAT-DEVELOPABLE &#34;DRY SILVER&#34; SHEET MATERIALS ARE STABILIZED AGAINST BACKGROUND DISCOLORATION UNDER USE CONDITIONS BY INCORPORATION OF SELECTED HALOGENCONTAINING ORGANIC OXIDIZING AGENTS, E.G. TETRABROMOBUTANE.

United States Patent US. (:1. sis-114.1 8 Claims ABSTRACT OF THE DISCLOSURE Light-sensitive heat-developable dry silver sheet materials are stabilized against background discoloration under use conditions by incorporation of selected halogencontaining organic oxidizing agents, e.g. tetrabromobutane.

This invention relates to light-sensitive heat-develop able sheet materials and in particular to the dry silver class of sheet materials described in US. Pat. No. 3,457,- 075. In one aspect the invention relates to the stabilization of such dry silver sheet materials, and to sheet materials thus stabilized.

Dry silver light-sensitive heat-developablesheet materials as described in said Pat. No. 3,457,075 comprise a combination of light-stable silver salt and reducing agent as the image-producing entity, and a small proportion of photosensitive silver halide in catalytic proximity to the silver salt as a means for increasing the rate of heat development of the combination. Upon exposure to a light-image followed by moderate heating, the light-exposed areas rapidly darken to provide a visible image. The unexposed areas remain virtually unchanged during normal heat development; but are found to darken gradually under subsequent prolonged or intensive heating or on extensive exposure to actinic radiation.

The photosensitive silver halide component of the sensitive sheet is most conveniently formed by reaction of an ionizable halogen compound with the light-stable silver salt. For example a suitable carrier such as paper or film is supplied with a first coating containing a silver salt and a second coating containing a reducing agent together with an ionizable halide, the second coating being applied under darkroom conditions. The halide ion reacts with a portion of the salt to form photosensitive silver halide in catalytic proximity with the remainder of the silver salt. The resulting sheet is photosensitive, and image areas, after exposure to light, are rapidly darkened by moderate heating, producing prints of excellent contrast. Prolonged exposure of background areas to light or to mild heating thereafter, i.e. under frequently experienced conditions of use, causes slow darkening of background areas resulting in reducedcontrast and print quality.

Surprisingly, it has now been found possible to drastically reduce or completely eliminate the tendency of the background areas of these dry silver sheet materials to darken in use, by incorporating in the sensitive coating controlled small proportions of selected organic oxidizing agents, and particularly of photosensitive halogencontaining organic oxidizing agents. Photosensitivity of the sheet is unchanged, and full image density is retained.

One compound which has been found highly effective 3,707,377 Patented Dec. 26, 1972 as a stabilizer is tetrabromobutane. It is a compatible colorless solid and imparts neither color nor opacity to the sensitive coating. It is non-volatile and is neither odoriferous nor fugitive. It is stable and does not decompose or react either in the coating solution or in the dried coating prior to exposure. However after the sheet has been expsed to light the compound, or its light degradation products, serves effectively to prevent or at least greatly retard backgrounding of dry silver sheet materials.

Various other organic oxidizing agents are similarly useful and may be substituted for the tetrabromobutane in whole or in part. Compounds wherein the halogens are bonded to the same or ladjacent aliphatic carbon atoms, such for example as hexabromocyclohexane and tribromoquinaldine, are substantially as effective as tetrabromobutane. On the other hand, oxidizing agents such for example as N-bromosucci'nimide, N-chlorosuccinimide and trichloromelamine are found to reduce the photosensitivity of the coating when incorporated therein and are therefore not contemplated for use in the practice of the invention. At the other extreme, compounds such as 1,8-dibromooctane and tetrachlorophthalic anhydride fail to stabilize the background areas to any useful extend and are therefore likewise eliminated.

The stabilizer compounds of this invention are found to respond to a modification of the benzidine test described by F. Feigl in his book Spot Tests In Organic Analysis. In the modified test a dilute solution of N,N'- diphenylben-zidine (0.1% in methanol) is first mixed with a solution of the compound (1% in aqueous acetone), in amount up tonne-fifth the volume of the latter if necessary, and observed for color formation. If the solution rapidly becomes significantly colored or darkened, the test is discontinued and the compound is characterized as overly reactive. If the solution remains substantially unchanged for several minutes at room temperature, it is then exposed in a small quartz vial to UV radiation (from a GE sunlamp at 6-10 inches) for onehalf hour and again examined. The absence of a distinct discoloration indicates the compound under test to be ineffective as a stabilizer. Compounds which remain colorless during and prior to exposure and which produce a distinct color during exposure are particularly useful as stabilizers for the purposes of this invention. Tetrabromobutane, for example, produces no color change during addition of the benzidine solution but gives a dark brown coloration on exposure of the solution to UV. Hexabromocyclohexane similarly gives no visible change in the first stage but develops a light brown color in the second. Tetrachlorobutane, which is of marginal utility, causes a barely visible discoloration in the first stage and produces a light brown color in the second stage.

The ability of the coatings and sheet materials of the invention to form dense images while avoiding subsequent backgrounding is influenced also by the quantity or concentration of the stabilizer material. With insufiicient stabilizer, an increase in background density may occur; Whereas an excess of stabilizer may reduce the photos-ensitivity or may prevent the development of a visible image. In general the quantity of tetrabromobutane required for stabilization of a dry silver coating based on silver behenate is found to be within the extremes of 0.025 to 0.14 mol of stabilizer per mol of total silver, or about 0.5 to about 3.5 mols of stabilizer per mol of silver halide.

Proportions of about one to about two mols of stabilizer per mol of silver halide are preferred.

The following examples, in which all proportions are in parts by weight unless otherwise noted, will further illustrate the practice of the invention.

silver'i behenate full soap in 37.5 parts of methylethyl ketone and 42.5 parts of toluene. The full soap is prepared from the sodium soap of commercial behenic acid and an equimolar amount of silver nitrate. The sheet is made more sensitive to green light by the incorporation of a spectrally sensitizing dye such as 3-allyl-5-[3-ethyl-(2- naphthoxazoylidene)ethylidene] l phenyl-2-thiohydantoin.

The composition is coated through a three mil orifice on two mil Mylar polyester film and dried. All operations beginning with the addition of mercuric bromide are conducted in the dark or under a red safelight.

A second coating composition is prepared by combining:

3% cellulose acetate butyrat'e solution in methylethyl ketone rnl 75 20% solution of polymethylmethacrylate in ethyl acetate ml Phthalazinone gm 1 2,2 methylenebis (4 methyl 6 tertiarybutylphenol) gm- 4 2% solution of phthalic acid in methyl alcohol ml. 1.5

This'-composition also is applied, over the previous coating, at the same orifice setting, and is then dried, to form a.control sheet. Other portions of the film coated with the first composition are separately coated with five gm. portions of the second composition to which is first added 150 mgm. of hexabromoethane or tetrabromobutane.

The three films are each exposed to a light image from a 650 watt FAB tungsten filament quartz iodine lamp through a photo transparency and are then briefly heated by contact with a heating platen for a time and at a temperaturesuflicient to develop a good image. In all cases a sharp black image on a clear background is obtained. The three prints are then passed through a thermographic copying 'machine where they are identically briefly exposed to intense radiant energy high in infra-red. The image on the control sample becomes blurred until it is completely illegible, and the background is discolored to a light buff. The film containing the hexabromoethane retains a legible image but with a light butf background. The film with the tetrabromobutane retains a legible. image and the background remains unaltered.

EXAMPLE 2 Silver behenate half soap, an equimolar mixture of silver behenate and behenic acid, is mixed in a ball mil with acetone and toluene to provide a smooth slurry. To the slurry; is added, in order, polyvinyl butyral resin binder, mercuric acetate, calcium bromide, and phthalazinone. The salts are added in solution in small amounts of methyl alcohol, and additional acetone is added as required for viscosity control. Tetrabromobutane stabilizer is finally added and mixed in, and the mixture is then coated an dense white paper and dried. All operations 4 beginning with addition of calcium bromide are conducted under darkroom or safelight conditions. The dry coating weighs 1.3 gm./sq. ft. and is represented by the following calculated percentage composition:

Half soap 8.75 Polyvinyl butyral 10.40 Phthalazinone 1.25 Mercuric acetate .03 Calcium bromide .05 Stabilizer .30

A further composition containing additional phthalazinone, sensitizing dyes, reducing agent, mineral filler and cellulose acetate resin binder dispersed or dissolved in acetone and methyl alcohol is prepared, coated over the first coatingpand dried. The added coating 'weighs 0.30 gm./sq. ft. and has the following calculated percentage composition:

Cellulose acetate 5.00

Reducing agent 3.50 Sensitizing dyes 0.001 Phthalazinone 0.60 Silica powder (Syloid 244) 0.7

Phthalazinone, i.e. 1-(2H)-phthalazinone, serves as a toner for the silver image. The reducing agent is 2,2- methylenebis-(4-methyl-6-tertiarybutylphenol). The dye mixturecontains one part of Victoria Blue B (CI. 729) and three parts of 3-allyl-5-[3-ethyl-(Z-naphthoxazoylidene)-ethylidene]-l-phenyl 2 thiohydantoin. The stabilizer i s 1,2.3,4-tetrabromobutane.

The finished sheet is compared with a control sheet of identical construction except for omission of the stabilizer. The two sheets are exposed to light from a tungsten filament through a step wedge and developed by holding against a polished metal drum maintained at 220 F. for about two seconds, i.e. suflicient to provide a dense black image on a White background. The sheets are permitted to remain exposed to artificial light in a well-lighted laboratory for one week and then tested for background density. The control sample has a light buff appearance and a measured optical density of 0.24. The stabilized sheet is white in appearance and exhibts a density reading of 0.13-0.14. The image areas on both sheets are an intense black.

In another test the two sheets are exposed to a lightimage and then heated for three seconds at 260 F. to develop an intense black visible image on a white background; The samples are then placed in a test chamber maintained at F. and 80% relative humidity, and exposed to tungsten filament light at 70 foot-candles, for 24 hours. The control sheet exhibits extensive darkening, to a distinct buff color, at the background areas; the stabilized sheet remains unchanged. No observable change is detected in the image areas.

EXAMPLE 3 A coating composition is prepared of the following components:

The silver soap mixture contains 10.83 parts of the equimolar blend of silver behenate and behenic acid in 30.23 parts of toluene and 58.94 parts of acetone. The 3.5 parts of terpene resin are added in solution in the 12 parts of toluene. The operations are conducted with due regard to protection from light. The composition is coated on a paper backing through a three mil orifice to provide a coating weight of approximately one gm. per square foot after drying.

A control sheet is similarly prepared except that the tetrabromobutane is omitted. The two sheets are exposed to a light image, and a visible image is developed by brief heating. Both samples provide a dense black image on a stable white background as first prepared. The printed sheets are allowed to remain exposed to artificial room light for 24 hours. At the end of this time the control sample evidences a distinct yellowing in the background areas, whereas the sample containing the tetrabromobutane stabilizer remains unchanged.

What is claimed is as follows:

1. Sheet material useful in imaging by a process involving exposure to a light-image followed by uniform heating and which thereafter is resistant to background discoloration, said sheet material including a stratum containing (a) photosensitive silver halide catalyst-forming means,

and

(b) heat sensitive reactant image-forming means including an organic silver salt oxidizing agent and a reducing agent for silver ion, the oxidation-reduction reaction of which to produce a visible change is accelerated by said catalyst, said photosensitive means being in catalytic proximity to said heat-sensitive means in quantity equal to at least about one-tenth mol percent based on said organic silver salt and suflicient to provide a gamma infinity value of at least about 0.5 when said stratum is exposed imagewise to said light-image and the image is then developed by uniform heating, and said stratum additionally containing, as a stabilizer,

(c) a photosensitive halogen-containing organic oxidizing agent characterized by its ability to respond to the modified benzidine test as described in the specification, i.e., by producing substantially no color in the first step and a distinct coloration in the second step, the amount of said stabilizer being between about 0.025 and 0.14 mol per mol of total silver and insufficient to cause any significant reduction in photosensitivity but adequate to prevent any substantial increase in background density.

2. The sheet material of claim 1 wherein the stabilizer contains a plurality of halogen atoms bonded to the same or adjacent aliphatic carbon atoms.

3. The sheet material of claim 2 wherein the halogen atoms are bromine atoms.

4. The sheet material of claim 3 wherein the stabilizer is 1,2,3,4-tetrabromobutane.

5. The sheet material of claim 3 wherein the stabilizer is hexabromoethane.

6. The sheet material of claim 3 wherein the stabilizer is tribromoquinaldine.

7. The sheet material of claim 3 wherein the stabilizer is 1,2,3,4-hexabromocyclohexane.

8. The sheet material of claim 2 wherein the stabilizer is present to the extent of between about one and about two mols per mol of said silver halide.

References Cited UNITED STATES PATENTS 3,547,648 12/ 1970 Sagawa 96-114.l 3,531,286 9/1970 Renfrew 96-1 14.1 3,515,559 6/l970 Druker 96-1 14.1

J. TRAVIS BROWN, Primary Examiner W. H. LOUIE, JR., Assistant Examiner US. Cl. X.R. 

